Situation:
The teacher enters the classroom and see a child sitting on another child who is lying on the floor of the classroom, holding his hands.
The teacher takes the child off of him yelling / scolding him and for dessert adds a penalty.
What the teacher did not see that half an hour before it, that "poor" child would not stop pestering and annoying the kid on top and only when it expired nervous he did what he did.
my questions are:
1. Is the teacher Correct behavior with the child?
2. Is there really a child on top Correct behavior
3. What to do with a child's frustration at not only plagued him he still got punished?
Thank a lot
and sorry for my english

This teacher should have the hootzpa to apologize to the misjudged child. And as for the little criminal that he was on top of, tell him to cease and desist from bullying his classmate. Then let the teacher check in weekly...EVERY WEEK, as to how their relationship is developing. He should even tell both of them to "GRADE" each others kindness towards them. From a A+, B-, C or from a 1 to 10 scale.

Right, but Rambam is not the only one on board. There are some others who say that something as a healthy anger does exist, that we shoudn't always smile when a situation asks for a controled anger. Expressing an angered and displeased face, while remaining calm within yourself is also another alternative.

This is not a question about what Chabad holds. I'd be interested in finding any source that says anger is a neutral middah. Rambam says no. In Chassidus, we hold anger akin to avodah zarah. But if there is a source that says otherwise, I'd be interested in seeing it.

FlyingAxe: Not all gevurah is bad, and not all chessed is good. Both can be appropriate when done in the right time and measure, or inappropriate when not done so. See here.

Even gevurah must be appropriate. For example, reprimanding a child is an act of gevurah. Reducing his grade for a Math homework because he didn't show his work as an incentive to do so is an act of gevurah (not always appropriate one, by the way). Showing one's anger is a sign of lack of control over one's emotions.

Furthermore, it is disrespect to a child who is, after all, another Jew. I don't understand how people find it permissible to act towards children (especially not their own children, i.e., strangers) in a way they would never act towards adults. (Yes, there are some corporate bosses who yell at their employees, etc. But I think most people would agree that such behavior is not appropriate for a civilized human being.)

Furthermore, we are talking about education here. The Rebbe, in a sicha commenting on Pirkei Avos, notes that we give examples of the animals we should learn from (in Pirkei Avos and in Shulchan Aruch), and these animals are not kosher. At the same time, the Rebbe himself instituted the campaign of making sure that non-kosher animals are not children's toys.

The difference is: one should learn from these animals, but not as a child. Children should be raised in purity and can learn from the animals when they are older.

Children are influenced especially easily, and things that they see in their childhood affect the way they will behave as adults (bechira and education/upbringing in teenage years and adulthood notwithstanding). So, by showing anger, a teacher shows the child that it is ok (or even appropriate) to get angry at another person and display the anger in public. He shows that it is appropriate to embarrass another person in public.

Also, I don't care if there is a practice of throwing salad at other people during farbrengens (or calling them fools), and if this practice goes back to Alter Rebbe's chassidim.

I assume you mean the Maamar Chazal (maybe based on the gemoro in Shabbos, maybe on the Zohar, כידוע השקו"ט בזה) which the Rambam quotes (not a chiddush of Chassidus, though Chassidus explains the connection (as do other seforim)).

I do not know where it would say anger is a "neutral midah", but there are cases that it is justified.